Microsoft's Internet Explorer browser again lost usage share last month to rivals Google and Apple, posting its largest decline since March, a Web analytics firm said Wednesday.

Usage share of Internet Explorer 6 (IE6), the nine-year-old browser even Microsoft wants to kill, fell by 1.3 percentage points during November, its biggest drop in eight months, to lead the decline.

According to Aliso Viejo, Calif.-based Net Applications, IE's total share fell by nine-tenths of a percentage point to 58.3%, the lowest level ever and the fifth month in a row that it lost ground.

Meanwhile, Google's Chrome and Apple's Safari made the sharpest gains, boosting their shares to 9.3% and 5.6%, respectively, both records.

But Microsoft preferred to talk about the silver lining -- the fall of IE6 to 13.7% of all browsers used worldwide last month -- rather than the continued weakening of its browser's total share.

"What we're trying to do is get people off IE6," said Roger Capriotti, the director of product marketing for IE, in an interview Tuesday. "We're happy with the [overall] numbers for this month, and we'll continue to focus [on getting people] on a modern browser, which is IE8 and then IE9."

Capriotti, who last summer said his job was to drive IE6 into extinction, a battle he said was "definitely winnable," also cited Net Applications' data to tout the decline of IE6 in business, one of the strongholds of the aged browser.

"We're excited to see IE6 usage in businesses worldwide at just 10.3%," Capriotti said, referring to data from Net Applications that is not made available to the public.

Net Applications regularly reviews month-end numbers with Microsoft and an unknown number of other browser providers, before posting a limited amount of information on the Web for the public.

Capriotti said the smaller IE6 share in businesses -- more than three percentage points lower than the worldwide figure -- proved that companies of all sizes were migrating to a more standards-compliant, more secure browser.

"It takes work to [move off IE6]," he acknowledged, "but many customers are telling us it's not as bad as they thought it was going to be."

If accurate, that should add to Microsoft's coffers, since one of the stumbling blocks to upgrading to Windows 7 is that it cannot run IE6 without some virtualized assistance, or an add-on like Unibrows. In a research note last month, Gartner said that IE6 compatibility problems will cause at least 20% of organizations to take longer than expected or spend more than they budgeted for their Windows 7 migration projects.

Not surprisingly, smaller companies -- which would likely rely on fewer custom Web applications -- have bailed on IE6 much faster than large corporations. According to Microsoft's mining of Net Applications' data, in companies with up to 500 employees, IE6 accounted for just 8% of the browsers used. In companies with more than 50,000 people, IE6 owned a 12% share.

"It's encouraging that businesses are upgrading en masse from IE6 to IE8 and other more modern browsers," said Vince Vizzaccaro, Net Applications' vice president of marketing. "It also allows for more recent standards to be in place across all modern browsers, making the browsing experience quicker, more consistent."

Capriotti declined to peg a timeline for IE6's next major milestone, a drop under 10%, saying, "I don't have a crystal ball, but we have a good trajectory going."

If IE6 continues to lose share at the average pace of the last three months, it will fall below 10% in May 2011. Microsoft has promised to support the browser with security updates through April 2014, however.

IE8 gained seven-tenths of a point, ending the month at 32.8%, while the beta of IE9 grew by a tenth of a percentage point to 0.4%. IE7, which has been closely associated with Windows Vista, lost share in November, falling four-tenths of a point to 9.5%.

Mozilla's Firefox and Opera Software's Opera also slipped last month, although both dropped by less than one-tenth of a point. Firefox accounted for 22.8% of all browsers, its smallest share since July 2009, while Opera held just 2.2% of the market.

Both Firefox and Opera have lost share in three of the last four months.

Net Applications calculates usage share from data acquired from the 160 million unique visitors who browse the 40,000 Web sites it monitors for clients. The company's November browser data is available on its site.